Nucleolar Dominance

Abstract

Abstract Nucleolar dominance is an epigenetic phenomenon that describes the formation of nucleoli, the sites of ribosome synthesis, at specific loci of chromosomes inherited from only one progenitor of a genetic hybrid. These loci, known as nucleolus organiser regions (NORs), consist of ribosomal RNA (rRNA) genes that are repeated in hundreds of copies. The molecular basis for nucleolar dominance is the silencing of rRNA genes at some NORs, but not others, a manifestation of a dosage control system that regulates the number of active rRNA genes depending on the cellular demand for ribosomes and protein synthesis. rRNA gene silencing involves cytosine methylation, histone post‐translational modifications and noncoding RNAs, but the precise mechanisms by which specific sets of rRNA genes are chosen for silencing are unclear. Key Concepts: In genetic hybrids, genes inherited from the different parents, or progenitors, can be differentially expressed. Nucleolar dominance in genetic hybrids involves the selective silencing of ribosomal RNA gene loci that span millions of base pairs on eukaryotic chromosomes. Ribosomal RNA gene silencing involves epigenetic modifications of chromosomal chromatin, including cytosine methylation and repressive histone post‐translational modifications. Nucleolar dominance is not a regulatory phenomenon unique to hybrids but a manifestation of a dosage control system that operates in all eukaryotes to control the number of active rRNA genes. Nucleolar dominance may result from inactivation of entire nucleolus organiser regions (NORs) rather than silencing decisions made one rRNA gene at a time.